1. Field of the Invention
This invention relates to a method for making thin films of polymer which have wide utility as gas separation membranes, insulating films and resist films for semiconductor circuits.
2. Description of the Prior Art
For the formation of polymer thin films, there is known a method wherein a solution of a membrane material such as a polymer in volatile solvent is cast on a liquid surface, followed by evaporating the solvent to obtain a thin film of the polymer. This method is described, for example, in Japanese Patent Publication No. 58-33086 (corresponding to U.S. patent application Ser. No. 356,514, filed May 2, 1973, now abandoned).
In this known method, a polymer solution is dropped on the surface of a support liquid in an area sectioned between two movable solid partition members which are placed in parallel relation. After the dropping, the members are moved to widen the section, thereby more spreading the solution on the liquid surface. In this state, the solvent is allowed to evaporate thereby obtaining a thin film of the polymer.
The manner of the casting slightly differs depending upon the surface treatment of the solid partition members. The case where the solid partition member has been subjected to water-repellent treatment on the surface thereof is particularly shown in FIG. 3. In the figure, there is schematically shown a water vessel 11 in which a support liquid 12 such as water is placed. Indicated at 13 are water-repellent solid partition members and at 14 is a polymer solution being cast. Since the members 13 are water repellent in nature, the support liquid is repelled with the members 13 and becomes depressed at portions contacting the members 13. The polymer solution 14 is dropped at the depressed portion whereupon it is spontaneously spread.
On the contrary, the case using the members 13 having a hydrophilic surface is shown in FIG. 4, in which like reference numerals as in FIG. 3 indicate like parts or members. Since the members 13 are hydrophilic on the surfaces thereof, the meniscus becomes concave as shown in the figure, thereby forming a concave liquid surface between the members 13. The polymer solution 14 is dropped at the concave liquid surface and is spontaneously spread over the liquid surface.
According to our experiments, it was found that the contact of the polymer solution 14 with the members 13 placed a limit on the uniformity in thickness of the resulting film.
Moreover, when the hydrophilic members 13 are used, the polymer solution 14 is collected in the depressed portion of the concave liquid surface under which it is cast. Immediately after the casting, the monomolecular layer spreads at the interface with the support liquid 12, under which it becomes almost impossible to spread a thin film of the polymer.
A further disadvantage involved in the prior art methods is that the interface with the support liquid is disturbed by the impact caused by the dropping of the polymer solution, giving an adverse influence on the uniformity of film thickness.